In the past few years there has been an increased interest in a new type of laser device called a vertical cavity surface emitting laser (VCSEL). Advantages of VCSEL devices are that the device is smaller, has potentially higher performance, and is potentially more manufacturable. These advantages are due in part from advances in epitaxial deposition techniques such as metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE).
VCSELs are formed by depositing a plurality of layers on a substrate to form the VCSEL. See, for example, U.S. Pat. No. 5,034,092, entitled "PLASMA ETCHING OF SEMICONDUCTOR SUBSTRATES", issued Jul. 23, 1991, assigned to the same assignee and included herein by this reference, and U.S. Pat. No. 5,256,596, entitled "TOP EMITTING VCSEL WITH IMPLANT", issued Oct. 26, 1993, assigned to the same assignee and included herein by this reference.
VCSELs generally include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL is driven by current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack.
The use of mirror stacks in VCSELS is well established in the art. Typically, mirror stacks are formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an AlAs/GaAs or AlGaAs/AlAs material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content in the layers. In conventional devices, the number of mirror pairs per stack may range from 20 to 40 to achieve a high percentages of reflectivity, depending on the difference between the refractive indices of the layers. The large number of pairs increases the percentage of reflected light.
Traditionally, the transmission distance of a multimode fiber based data link has been limited to 300-500 meters because of the limitation of dispersion effects. Off-set launch methods have been used to allow less modes in the fiber to be excited, therefore reducing the dispersion effect. Offset launch methods are utilized because the light source operates in a single spatial mode. With a decrease in the dispersion effect, data can be transmitted across a longer distance. However, how much off-set is needed, and how to control the off-set uniformity from link to link in manufacturing is a problem.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved method for achieving greater distance data transmission based on multimode fibers utilizing a donut mode VCSEL.
Donut mode VCSELs operate to excite only higher order modes in the fibers without off-set launching. Through the excitation of less modes in the fibers, greater transmission distances can be achieved. In addition, donut mode VCSELs operate spectrally in a single mode. This characteristic can be utilized to achieve high speed data communication based on wavelength division multiplexing technology by combining a group of VCSELs or an array of VCSELs with each VCSEL operating at its own preset wavelength. It is therefore an object of the present invention to propose a method to achieve wavelength division multiplexing by using spectrally single mode but spatially donut mode VCSELs.
Another object of the invention is to provide a reliable method for achieving wavelength division multiplexing that allows for a large transmission distance.
Yet another object of the invention is to provide for a method for achieving wavelength division multiplexing in which cross-talk between neighboring channels is maintained at a minimum.
Another object of the present invention is to provide for a method of achieving wavelength division multiplexing in which a spatially donut mode and spectrally single mode VCSEL allows for greater system margin in wavelength division multiplexing and de-multiplexing.